Potential of a pH-Stable Microporous MOF for C2H2/C2H4 and C2H2/CO2 Gas Separations under Ambient Conditions

Cost-effective adsorption-based C 2 H 2 /C 2 H 4 and C 2 H 2 /CO 2 gas separations are extremely important in the industry. Herein, a pH-stable three-dimensional (3D) metal–organic framework (MOF), IITKGP-25, possessing exposed functional sites is presented, which facilitates such separations with excellent ideal adsorbed solution theory (IAST) selectivity (4.61 for C 2 H 2 /C 2 H 4 and 3.93 for C 2 H 2 /CO 2 ) under ambient conditions (295 K, 100 kPa, 50:50 gas mixtures) and a moderate affinity toward C 2 H 2 (26.6 kJ mol –1 ). Interestingly, IITKGP-25 can maintain structural integrity in water and in aqueous acidic/alkaline (pH = 2–10) medium because of the higher coordination numbers around the metal center and the hydrophobicity of the ligand. The adsorption capacity for C 2 H 2 remains unchanged for a minimum of up to five consecutive cycles and 15 days of exposure to 97% relative humidity, which are the prerequisites of an adsorbent for practical gas separation application. Density functional theory (DFT) calculations reveal that the open Cd(II) sites and carboxylate oxygen-coordinated Cd(II) corner of the triangle-shaped one-dimensional (1D) channel are the enthalpically more preferred binding sites for C 2 H 2, which stabilize the adsorbed C 2 H 2 through nonlocal stronger H-bonding and also pπ–dπ and CH−π interactions.